1. Field of the Invention
This invention is directed to apparatus for identifying true changes in state of electrical contacts which tend to bounce and for time stamping such changes. The invention has particular application in identifying and time stamping true changes in the state of the contacts in protective relays in electric power distribution systems.
2. Background Information
In many applications, it is desirable to record the time, referred to as time stamping, of electrical contact activity. Bouncing contacts pose a problem for time stamping. Waiting until a contact stabilizes before time stamping may be too inaccurate in some applications, and time stamping upon initial contact activity, though chronologically precise, may result in noise time stamping as well as multiple time stamps for a single contact toggle.
One type of apparatus where contact bounce is a particular problem is protective relays for electric power distribution systems. These relays include contacts operating a trip circuit to open a circuit breaker in response to the detection of fault currents in a protected section of a power line, and often also include contacts in alarm circuits, pilot circuits and for other functions. Such contacts tend to bounce on both opening and closing and may open and close in response to disturbances, such as may be caused by lightning.
It is desirable to accurately know the state of many of these protective relay contacts and to know within milliseconds the time of any change in the state of the contacts.
There is a need therefore for apparatus which accurately and reliably determines the state of electrical contacts which tend to bounce.
There is a further need for such apparatus which accurately and reliably time stamps contact activity.
There is an additional need for such apparatus which is flexible and can be easily adapted for contacts with different characteristics.